Evolution of surface morphology, crystallite size, and texture of WO3 layers sputtered onto Si-supported nanoporous alumina templates

Thin Al films sputtered on n-silicon wafers were anodized in two potentiodynamic steps in 0.4 M tartaric acid at 210 V and 0.4 M malonic acid at 95 V to prepare nanoporous alumina templates of type A and B, respectively. Processes at the Al/n-Si interface were closely examined to avoid field-assisted alumina dissolution during anodizing. WO3 semiconductor layers were radio-frequency magnetron sputtered onto the as-anodized and pore-widened templates. It was revealed that the sputtered WO3 layers covered the entire surface of type A templates while only partly penetrating into the widened pores of type B templates. The WO3 layers crystallized at 350 degrees C, with monoclinic phase of P2(1)/n symmetry and an average crystallite size of 28.0 nm (type A) and 32.5 nm (type B). Amorphous alumina in the Al2O3/n-Si systems did not crystallize up to 1200 degrees C. The common trend for the WO3/Al2O3/n-Si systems was a preferential growth of WO3 nanocrystallites in the (100) direction, the texture increasing with further pore widening. Besides, the crystal texture in the WO3 layer was systematically higher on type B templates. The link between the structural features and the gas-sensing ability of the WO3/Al2O3/n-Si systems has been discussed. (C) 2008 The Electrochemical Society.